Efficient hash table access with constructed keys - data-structures

Question

Common Lisp allows any lisp object to serve as a hash table key. But what if you only want to use part of an object as the key. For example, in
(defstruct action
(name nil :type symbol)
(parameters nil :type list)
(time 0 :type fixnum)
(database nil :type hash-table))
the time slot is inappropriate for equalp hashing situations. What is a good strategy for accessing a hash table using a (partial) lisp object as key? One approach might use a key like (list (action-name act1) (action-parameters act1) (action-database act1)), but this seems rather inefficient. Another approach might create a substructure to the action defstruct with just the three appropriate slots, and use that substructure as a key, but this seems somewhat ad-hoc just for the purpose of hash table access. Are there other methods that could work better?

Answer 1

I will use Common Lisp library from here, as for example
cl-custom-hash-table
Then going to your code, first when you create an action like this:
CL-USER> (setq action-1 (make-action
:parameters '(1 2 3)
:time 45))
produce this error:
The value NIL is not of the expected type HASH-TABLE.
[Condition of type TYPE-ERROR]
so you need to change your definition to something like this:
CL-USER> (defstruct action
(name nil :type symbol)
(parameters nil :type list)
(time 0 :type fixnum)
(database (make-hash-table) :type hash-table))
ACTION
CL-USER> (setq action-1 (make-action
:parameters '(1 2 3)
:time 45))
#S(ACTION :NAME NIL :PARAMETERS (1 2 3) :TIME 45 :DATABASE #<HASH-TABLE :TEST EQL size 0/60 #x3020012E708D>)
Then you should define a function for equal time or what ever you need as follow:
accesing to the data
CL-USER> (action-time action-1)
45
create another action
CL-USER> (setq action-2 (make-action
:parameters '(1 2 3)
:time 102))
#S(ACTION :NAME NIL :PARAMETERS (1 2 3) :TIME 102 :DATABASE #<HASH-TABLE :TEST EQL size 0/60 #x3020012FE58D>)
create the function for testing
CL-USER> (defun equal-actions-by-time (a1 a2) (= (action-time a1) (action-time a2)))
EQUAL-ACTIONS-BY-TIME
define the hash-function:
CL-USER> (defun hash-action (a) (action-time a))
HASH-ACTION
create your hash
CL-USER> (define-custom-hash-table-constructor make-action-hash :test equal-actions-by-time :hash-function hash-action)
MAKE-ACTION-HASH
CL-USER> (defparameter *foo-hash* (make-action-hash) "variable for stackoverflow")
*FOO-HASH*
try it:
CL-USER> (setf (gethash action-1 *foo-hash*) 1
(gethash action-2 *foo-hash*) 10)
10
CL-USER> (gethash action-1 *foo-hash*)
1
T
You can avoid using a library if the distribution will work in implementations that support custom TEST/HASH functions natively, if not you can use with-custom-hash-table
In the optimus case you can work as follow:
CL-USER> (defparameter *foo-hash* (make-hash-table :test 'equal-actions-by-time :hash-function 'hash-action))
*FOO-HASH*
CL-USER> (setf (gethash action-1 *foo-hash*) 1
(gethash action-2 *foo-hash*) 10)
10
CL-USER> (gethash action-1 *foo-hash*)
1
T